DELETERIOUS RHIZOSPHERE BACTERIA - AN INTEGRATING PERSPECTIVE

The rhizosphere of plants is usually occupied by both deleterious rhiz obacteria (DRB), which may inhibit plant growth, and plant growth prom oting rhizobacteria (PGPR). Root exudates play a key role in determini ng host specific interactions with, and the composition of, the rhizob acterial population. A degree of coevolution between rhizobacteria and plants is implied. DRB have been implicated in yield declines associa ted with continuous monoculture. DRB inhibit growth primarily through production of phytotoxins but phytohormone production, competition for nutrients and inhibition of mycorrhizal function may also occur. The effect of individual isolates of rhizobacteria on plants can fluctuate from growth inhibition to growth promotion, according to environmenta l conditions, the host genotype and mycorrhizal status. Furthermore, r hizobacteria have the potential to act as both DRB and PGPR through si multaneous inhibition of plant growth and control of fungal plant path ogens. Therefore classification of rhizobacteria as either DRB or PGPR is somewhat equivocal and should be based upon their net effects on p lant growth according to circumstance. The capacity for fluctuation in the phytogenic function of rhizobacteria presents a challenge for man agement of DRB and manipulation of the rhizosphere to improve the sust ainability of agriculture.